Many people subconsciously make the mistake of only seeing the issues concerning 9/11 in black and white, as opposed to shades of gray. This is known as the black-or-white fallacy. In this case, the false dilemma is: 9/11 was either carried out by Al-Qaeda or it was "an inside job."
Just because the evidence suggests that rogue elements of US and other international intelligence agencies were involved doesn't mean bin Laden and Al-Qaeda hijackers weren't involved.